1. Field of the Invention
The present invention relates to a laser resonator for exciting a laser gas in a discharge tube to emit a laser beam, and more particularly to a multiple-folded laser resonator.
2. Description of the Related Art
Gas lasers such as carbon dioxide lasers are capable of producing high output emissions at high energy efficiencies. Since such gas lasers have good beam characteristics, they can machine workpieces to complex shapes at high speeds under the control of numerical control systems. Therefore, the gas lasers are widely used as machining apparatus in the art.
Many gas lasers employ multiple-folded gas laser resonators because they need a wide discharge range in order to produce high output emissions.
FIG. 3 of the accompanying drawings schematically shows a conventional laser resonator. The illustrated laser resonator is designed for use in a multiple-folded fast-axial-flow CO.sub.2 gas laser combined with a machining nozzle.
As shown in FIG. 3, the laser resonator has a total of four discharge tubes 211.about.214, with the discharge tubes 211, 212 being connected in series with each other by a gas outlet discharge tube holder 231 and the discharge tubes 213, 214 being connected in series with each other by a gas outlet discharge tube holder 232. The interconnected pair of discharge tubes 211, 212 and the interconnected pair of discharge tubes 213, 214 are disposed parallel to each other. The opposite ends of the interconnected pairs of discharge tubes 211.about.214 are fixed in position by gas inlet discharge tube holders 233.about.236. The gas outlet discharge tube holders 231, 232 have respective gas outlets 241, 242, and the gas inlet discharge tube holders 233.about.236 have respective gas inlets 243.about.246.
The laser resonator also has a rear mirror 222 mounted on an end of the discharge tube 214, folding mirrors 223, 224 mounted on respective ends of the discharge tubes 211, 213, and an output mirror 221 mounted on an end of the discharge tube 212. The rear mirror 222, the folding mirrors 223, 224, and the output mirror 221 will hereinafter be also referred to as internal optical mirrors. The four discharge tubes 211.about.214 are interconnected by a discharge tube coupling 250 disposed between the folding mirrors 223, 224. The laser resonator of the above construction functions as a single Fabry-Perot resonator.
A laser gas to be introduced into the discharge tubes 211.about.214 is compressed by an air blower of the laser and then sent to a circulatory system. In the circulatory system, the laser gas is cooled to a desired temperature, which is substantially equal to an ordinary temperature, by a heat exchanger which deprives the laser gas of the heat produced when it was compressed. The laser gas that is maintained at the constant temperature is then introduced from the gas inlets 243.about.246 into the discharge tubes 211.about.214.
The introduced laser gas flows in the discharge tubes 211.about.214 from the gas inlet discharge tube holders 233.about.236 toward the gas outlet discharge tube holders 231, 232 at a speed of about 200 m/sec. The fast-flowing laser gas is excited in the discharge tubes 211.about.214 by the application of a high-frequency discharge. When the laser gas is excited, it emits a laser beam 201 at a constant wavelength from laser gas molecules. The laser beam 201 is amplified by resonating between the rear mirror 222 and the output mirror 221, and a portion of the amplified laser beam 201 is outputted as a high-power laser beam 202 from the output mirror 221.
The internal optical mirrors are exposed to the laser gas while the laser gas resonator is in operation. The laser gas that circulates in the circulatory system contains foreign matter such as dust, oil mist, etc. delivered from mechanical components such as the air blower. As a result, the reflecting surfaces of the internal optical mirrors tend to be contaminated by the foreign matter contained in the laser gas, and the contaminants on the internal optical mirrors are liable to lower the output power of the laser beam 202. Therefore, it is necessary to clean the contaminated internal optical mirrors at periodic intervals. In order to clean the contaminated internal optical mirrors, they have to be removed from the laser gas resonator. After being cleaned, the internal optical mirrors are installed in place in the laser gas resonator.
The optical systems in the laser gas resonator need highly strict adjustments. Therefore, when the internal optical mirrors are removed and installed again, these optical systems have to be readjusted for alignment of their optical axes. The optical axis readjustment of the optical systems is apt to cause the output laser beam to be slightly shifted off its proper optical axis. Since the shifted output laser beam is displaced out of alignment with the center of the machining nozzle, the optical axis of an external optical system which guides the output laser beam to the machining nozzle has to be readjusted. The readjustment of those optical systems is a timing-consuming task.
The laser gas which circulates in the parallel pairs of discharge tubes 211.about.214 tends to suffer pressure differences developed due to different tube diameters, tube lengths, and so on. The pressure differences cause the laser beam to flow in the discharge tube coupling 250, and the laser beam flow in the discharge tube coupling 250 aggravates the contamination of the reflecting surfaces of the folding mirrors 223, 224, which require frequent cleaning.
As described above, the conventional laser resonator is disadvantageous in that since the reflecting surfaces of the internal optical mirrors can be contaminated easily, the internal optical mirrors have to be cleaned frequently, and since the internal optical mirrors need to be detached when they are to be cleaned and attached again after they are cleaned, and also need to be readjusted each time they are cleaned, the cleaning process is complex and time-consuming.